The present invention relates to a fixing device for electrophotographic copying machines, and more particularly to a fixing device in which an offset preventing liquid is applied to the surface of a fixing roller.
With electrophotographic copying machines, the latent electrostatic image formed on a photconductive drum is converted by a developing unit to a toner image, which is then transferred onto copy paper by a transfer unit. The copy paper is pressed and heated by a fixing device comprising a pair of fixing rollers, whereby the toner image is fixed to the copy paper.
As shown in FIG. 1, the fixing device according to the prior art usually comprises an upper fixing roller 10 having a heater 11 in its interior and a thin silicone rubber coating 12 over the outer periphery thereof for preventing offset of the toner image during fixing, and a lower fixing (press) roller 13 having a thick silicone rubber covering 14 over its outer periphery. Upper fixing roller 10 is held in pressing (nip defining) contact with lower fixing roller 13, e.g., by a spring (not shown), to provide a nip of increased width and to produce an enhanced fixing effect. Silicone oil 15, which serves as an offset-preventing liquid, is fed by a felt 16 to an applicator roller 17, which in turn applies the oil to the upper fixing roller 10 to assure prevention of offset and separation of the copy paper 18 from the upper fixing roller 10. Copy paper 18 is fed to the nip between the upper and lower fixing rollers 10 and 13 to heat-fix the toner image thereto by the heater and is thereafter delivered onto a tray.
The fixing device described above has the following drawbacks.
(1) The silicone oil 15 for preventing offset, when applied to silicone rubber coating 12, swells the rubber coating and thereby results in a larger outside diameter of roller 10, causing the copy paper to travel at an increased speed and thus bringing the paper out of timed relation with the other paper feeding mechanisms. This causes damage to the copy paper or impairs the toner image thereon.
(2) With the rotation of upper fixing roller 10, the offset preventing liquid is automatically fed to this roller since felst16 is in contact with applicator roller 17 at all times. The device therefore consumes large quantities of silicone oil 15, which leads to a higher operating costs, irrespective of whether or not the silicone rubber coating is used.
(3) Toner particles, paper particles, etc., adhere to felt 16 during use, while felt 16 is merely in loose contact with applicator roller 17, so that the felt is likely to apply the silicone oil unevenly to the applicator roller. The silicone rubber coating and covering 12 and 14 will then be swollen unevenly, possibly causing damage to copy paper 18 or impairing the toner image thereon.
These drawbacks can be eliminated if silicone oil 15 is applied to upper fixing roller 10 only in a minimum amount needed for the prevention of offset as disclosed, for example, in U.S. Pat. No. 4,045,165.
Whereas this is useful for the prevention of offset, the exceedingly small amount of silicone oil 15 will permit the fixed copy paper 18 to travel with upper fixing roller 10 with its leading end fitting to the roller without being properly separated thereform by a separator pawl (not shown) subsequently disposed.
On the other hand, an attempt to assure proper separation invariably entails the foregoing drawbacks.
We have conducted various investigations on the drawbacks of the conventional fixing device described above with the following findings:
(1) Silicone oil, even if applied in a minimum quantity, is useful for the prevention of offset.
(2) The copy paper, once separated at its leading end only, can be wholly separated automatically without the necessity of using silicone oil specifically for the separation.